Hemorrhagic fever viruses can only be diagnosed by conventional methods when the infection has spread to the blood the patient (viremia, around 8 days after infection). We set out to determine if the transcriptional dynamics of the circulating immune cells (PBMCs) in non-human primates infected with the Lassa virus could be used as an ealier indicator of infection. We identified a strong signal of infection that appear almost 5 days earlier (3dpi) than the one detected by conventional methods.